1. Field of the Invention
The invention relates generally to a deceleration control apparatus for a vehicle. More specifically, the invention relates to a deceleration control apparatus and method for a vehicle that applies a deceleration, which is equal to or smaller than a guard value, to the vehicle based on a vehicle running environment parameter (e.g. whether there is a curve in a road ahead of the vehicle, and a distance from a preceding vehicle).
2. Description of the Related Art
Performing deceleration control on a vehicle (for example, by applying an automatic brake or shifting to a lower gear of a transmission) based on a vehicle running environment parameter (for example, whether there is a curve in a road ahead of the vehicle, and a distance from a preceding vehicle) is known. Such a technology is described, for example, in Japanese Patent Application Publication No. JP-A-2000-145937. According to this publication, an appropriate speed for a vehicle to go round a curve in a road ahead of the vehicle is calculated before the vehicle enters the curve, based on road information that has been read from a navigation-processing unit. Based on the calculated appropriate speed, and the position and the speed of the vehicle, the necessity of deceleration is determined in advance. When it is determined that deceleration is necessary, shift control to select a predetermined gear is executed in response to a deceleration operation by the driver, such as release of an accelerator pedal or application of a brake pedal, and the gear is shifted down, for example, from fourth gear to third gear, or from third gear to second gear. Thus, a downshift is performed at a time at which the vehicle is at an appropriate position, and which accommodates the deceleration intention of the driver.
Applying a deceleration, which is equal to or smaller than a guard value (i.e., the maximum value of deceleration that a deceleration control apparatus for a vehicle can set), to a vehicle based on a vehicle running environment parameter is known. Setting a larger guard value makes it possible to apply a larger deceleration to the vehicle. In this case, the deceleration control produces sufficient effects. Meanwhile, the driver tends to depend on the control, resulting in negligent operation of the vehicle. In order to avoid this, the guard value needs to be set to such a value at which the driver feels the sufficient effect of the control but does not depend on the control. However, such a value varies depending on the driving style (or preference) of the driver. Accordingly, it is difficult to set a guard value applicable to every driver. Therefore, the guard value has been set to a sufficiently small value at which almost all the drivers do not depend on the control.